The rapid development of electronic devices has increased market demand for electrochemical devices such as fuel cells and battery systems. In response to the demand for battery systems in particular, practical, rechargeable lithium batteries have been actively researched. Lithium-ion batteries are particularly useful for many portable electronic devices. Lithium-ion batteries employ highly chemically reactive components to provide electrical current. These systems are typically based on the use of lithium metal, lithiated carbon, or a lithium alloy as the negative electrode (anode) and electroactive transition metal oxides as the positive electrode (cathode).
Lithium-ion batteries are constructed from one or more electrochemical cells connected in parallel or series. Such cells have consisted of a non-aqueous lithium ion-conducting electrolyte composition interposed between electrically-separated and spatially-separated, positive and negative electrodes. The electrolyte composition is typically a liquid solution of a lithium salt in a nonaqueous, aprotic organic solvent. A mixture of two or more organic solvents often is used.
The selection of electrolyte solvents for rechargeable lithium batteries is important for optimum battery performance and safety and involves a variety of different factors. However, long-term chemical stability in the presence of the charged positive and negative electrodes, ionic conductivity, safety, and wetting capability tend to be important selection factors in high volume commercial applications.
Long-term chemical stability requires that an electrolyte solvent be intrinsically stable over the battery's range of operating temperatures and voltages and also that it be either unreactive with electrode materials or that it contribute to effectively forming a passivating film with good ionic conductivity on the electrodes. Ionic conductivity requires an electrolyte solvent that effectively dissolves lithium electrolyte salts and facilitates lithium ion mobility. From the viewpoint of safety, the characteristics of low volatility, low flammability, low combustibility, low reactivity toward charged electrodes, passivating characteristics, and low toxicity are all highly desirable. It is also desirable that the battery's electrodes and separator be quickly and thoroughly wetted by the electrolyte solvent, so as to facilitate rapid battery manufacturing and optimize battery performance.
Aprotic liquid organic compounds have been the most commonly used electrolyte solvents for lithium batteries. Often, compounds such as carbonic acid esters (carbonates) have been used, as these compounds typically share the desirable properties of low reactivity with the positive electrodes operating at less than about 4.4V vs. Li+/Li, low reactivity with lithium-containing negative electrodes, and a thermodynamically favorable solvation interaction with lithium salts, which results in the electrolyte composition having a high ionic conductivity.
The most commonly used aprotic organic electrolyte solvents for use in lithium batteries include cyclic carbonates such as ethylene carbonate and propylene carbonate, cyclic esters of carboxylic acids such as γ-butyrolactone, linear carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, cyclic ethers such as 2-methyltetrahydrofuran and 1,3-dioxolane, linear ethers such as 1,2-dimethoxyethane, amides, and sulfoxides. A mixed solvent is often preferred in order to balance, or tailor, the desired properties of the electrolyte composition such as high dielectric constant and low viscosity.
Drawbacks to the use of conventional lithium battery electrolyte solvents are generally related to their properties such as low boiling points and high flammability or combustibility. Some solvents, such as ethylene carbonate and propylene carbonate, have boiling points above 200° C. However, many electrolyte solvents have boiling points that are substantially lower and have flash points less than 30.2° C. (100° F.). Such volatile solvents can ignite during catastrophic failure of a fully or partially charged battery that has undergone, for example, a rapid discharge due to a short circuit. Additionally, volatile solvents present difficulties in the preparation and storage of electrolyte compositions as well as in the addition of the electrolyte composition to the battery during the manufacturing process. Also, many conventional battery electrolyte solvents are reactive towards charged electrodes at elevated temperatures, which can result in thermal runaway under abuse conditions. In fact, recent news reports noted that overheating and even spontaneous combustion of secondary batteries have led to product recalls.